Edges Of Graphene Sheets Research Articles

Condensed water on superhydrophobic carbon films

Nanostructured carbon materials, including carbon nanotubes, nanofibers, and nanowalls, exhibit a wide range of interesting properties dictated by the many different bonding configurations. Many of these materials can possess superhydrophobic behavior when water drops are placed on their surfaces: these drops have high contact angles and can roll freely on the surfaces, which is desirable for self-cleaning. In this work, we prepared porous carbon films using a microwave plasma enhanced chemical vapor deposition technique. These films showed superhydrophobicity with contact angle of 150°, which was explained by the synergetic effect of the highly rough surface combined with the hydrogen terminated edges of graphene sheets. However, the condensed water drops can behave differently: the drops did not roll readily. This behavior mimicked that of water on lotus leaves and further demonstrated that the reported superhydrophobic behavior is a function of how the water gets on to the surfaces.

Scanning atom probe study of graphite nanofibers

Field emission characteristics of graphite nanofibers (GNFs) are studied by operating a scanning atom probe as a field emission microscope. The Fowler-Nordheim plot indicates that the work function of GNF is larger than that of carbon nanotubes and is comparable to tungsten. Field emission patterns indicate that electrons are emitted from the edges of graphene sheets in the direction perpendicular to the GNF axis. The ratio of the field emission voltage and that of the field evaporation voltage implies that the binding between the graphene sheets is weak. The field evaporated cluster ions of carbon and hydrogen indicate that hydrogen is not distributed randomly but is forming the characteristic carbon-hydrogen cluster, C23H2. The proposed structure of the cluster is the triangularly arranged six hexagonal cells with hydrogen terminated two carbon atoms and an extra carbon atom.

Structures and electronic properties of surface/edges of nanodiamond and nanographite

The structure, electronic and magnetic properties of nanodiamond and nanographite/nanographene are investigated. Detonation nanodiamond particles that are covered with amorphous graphitic composites are hydrothermally treated to remove the graphitic surface composites and to terminate the surface carbon atoms with hydrogen. The number of localized spins of dangling bonds and the hydrogen concentration increase upon the increase in the hydrothermal treatment time up to 40 h. Above 40 h, both drop discontinuously, a surface structural reconstruction was suggested. The creation of dangling bonds and an incomplete hydrogenation of the surface carbon atoms destabilize the surface, resulting in the structural reconstruction. Nanodiamond particles are thermally converted to nanographite/nanographene. A single nanographene sheet is successfully prepared by heat-treating nanodiamond particles. The edge of graphene sheet with its edge carbon atoms being hydrogen-terminated is investigated by UHV-STM/STS. Zigzag edges are found to have non-bonding π-state of edge origin, in good agreement with theoretical prediction.

On the Chemical Nature of Graphene Edges: Origin of Stability and Potential for Magnetism in Carbon Materials

Heretofore disconnected experimental observations are combined with a theoretical study to develop a model of the chemical composition of the edges of graphene sheets in both flat and curved sp(2)-hybridized carbon materials. It is proposed that under ambient conditions a significant fraction of the oxygen-free edge sites are neither H-terminated nor unadulterated sigma free radicals, as universally assumed. The zigzag sites are carbene-like, with the triplet ground state being most common. The armchair sites are carbyne-like, with the singlet ground state being most common. This proposal is not only consistent with the key electronic properties and surface (re)activity behavior of carbons, but it can also explain the recently documented and heretofore puzzling ferromagnetic properties of some impurity-free carbon materials.

Characterization of Aromatic Compound Sorptive Interactions with Black Carbon (Charcoal) Assisted by Graphite as a Model

Molecular interactions controlling the sorption of pollutants to environmental black carbons (soot, charcoal) are not well-resolved. Sorption of a series of aromatic compounds was studied to wood charcoal and nonporous graphite powder as a model adsorbent. Issues of concern were the possible involvement of pi-pi electron donor-acceptor (EDA) interactions of electron-poor and electron-rich solutes with the graphene (polycyclic aromatic) surface and size exclusion effects. Sorption of pi-acceptors, benzonitrile (BNTL), 4-nitrotoluene (MNT), 2,4-dinitrotoluene (DNT), and 2,4,6-trinitrotoluene (TNT), and to a lesser extent pi-donor solutes, naphthalene (NAPH) and phenanthrene (PHEN), was greater than predicted by hydrophobic driving forces in accord with their acceptor or donor strength. Hydrophobic effects were estimated using a concentration-dependent free energy relationship between adsorption and partitioning into an inert solvent (n-hexadecane or benzene) for a non-donor/non-acceptor calibration set (benzene and chlorinated and methylated benzenes). Molecular complexation between acceptors and model graphene donors, NAPH, PHEN, and pyrene (PYR), in chloroform and benzene was tracked by ring-current induced upfield shifts in the 1H NMR spectrum and by charge-transfer bands in the UV/visible spectrum. The EDA component of graphite-water adsorption for the acceptors correlated with the NMR-determined complexation constant with the model donors in chloroform, which, in turn, correlated with pi-acceptor strength (TNT > DNT > MNT > BNTL) and pi-donor strength (PYR > PHEN > NAPH). Charcoal-graphite isotherms calculated from charcoal-water and graphite-water isotherms indicated molecular sieving effects on charcoal for tetrasubstituted benzenes (tetramethylbenzenes and TNT) and some trisubstituted benzenes (1,3,5-trichlorobenzene, possibly DNT). When steric effects are taken into account, the order in adsorption among acceptors was qualitatively similar for graphite and charcoal. The results suggest pi-pi EDA interactions of the acceptors-and possibly donors, although the calibration set may underestimate the hydrophobic effect for fused ring systems-with both graphite and charcoal surfaces. For graphite, it is postulated that pi-acceptors interact with electron-rich regions of the basal plane near edges and defects and that pi-donors interact with electron-depleted regions further away. A similar mechanism may operate on the charcoal but would be modified by the (mostly) electron-withdrawing effects of 0 functionality on the edges of graphene sheets.

Supercapacitors Prepared from Melamine-Based Carbon

The electrochemical performance of supercapacitors made of a carbon material with a moderate amount of nitrogen atoms embedded in a carbon matrix is reported. Melamine was polymerized in the interlayer spaces of mica and afterward carbonized at various temperatures between 650 and 1000 °C. Elemental analysis and an XPS study showed that the nitrogen content of samples stabilized at 250 °C for 4 h prior to carbonization was generally higher if compared to their nonstabilized counterparts and that the nitrogen species were located preferably at the edges of graphene sheets. To understand the relationship between the capacitive performance and the porosity of stabilized and nonstabilized samples, the nitrogen adsorption/desorption method was also employed. Supercapacitors with the electrodes manufactured from these carbon materials showed a very good capacitive performance in 1 M sulfuric acid. The maximum gravimetric specific capacitance of 204.8 F g-1 was obtained from a sample carbonized at 750 °C. Specif...

A theoretical study on storage states of Li ions in carbon anodes of Li ion batteries using molecular orbital calculations

Semi-empirical molecular orbital calculations were carried out to clarify storage states of Li ions in amorphous carbon anodes of Li ion batteries. Storage states of Li ions between two graphene sheets were investigated and a favorable structure for a carbon anode to produce large reversible and small irreversible capacities is discussed. A polycyclic hydrocarbon molecule, C 54H 18, was used as a model of a graphene sheet. Relations between the interlayer distance of two graphene sheets and the storage state of Li ions were investigated, and preferable interlayer distances for specific numbers of Li ions were estimated. In particular, storage states with all Li ions on the basal area were treated, because the amount of basal carbons should be larger than that of edge carbons. The charge distribution of Li ions was also investigated. Calculated results suggested that a storage state in which a double Li ion layer was formed was preferable to achieve a larger capacity than the theoretical maximum capacity of graphitic carbons (372 mAh/g) and to reduce hysteresis in the charge–discharge process. Moreover, suitable distance between edges of graphene sheets to prevent the intercalation of electrolyte species was discussed. A recommended structure of carbon anodes suitable for the double Li ion layer storage and prevention of the intercalation of electrolyte species is proposed.